1. Field of the Present Invention
The present invention generally relates to capacitive discharge in an electronic system and more specifically to a circuit for rapidly eliminating charged stored in a computer system during a temporary loss of system power.
2. History of Related Art
Microprocessor based computing systems are widely used for personal, business, scientific, and government applications. The price-to-performance ratio of these machines has improved so dramatically in the 1990's that microprocessor based computers are now used for tasks previously reserved for workstations and mainframe computers such as performance demanding applications and large network controllers. The increased expectations placed on these machines by consumers have forced manufacturers of personal computers and other similar microprocessor based machines to devote increased attention to reliability issues. Consumers who rely on microprocessor based machines to run entire networks, maintain financial information including payroll, accounts receivable, accounts payable, and other critical functions require essentially zero down time.
Many modem computer systems include power management modules designed to minimize and monitor the power consumed by the computer system. Power management modules may include functionality designed to place system components in low power or standby mode if a particular device experiences no activity for a specified duration. While these modules are highly desirable to reduce the tremendous power consumed by high performance computer systems, the power management system adds complexity to the computer system. In addition to power management modules, typical computer systems include a variety of subsystems in addition to the core circuitry comprised of the microprocessor and its support devices. Each of these core systems and sub-systems includes thousand of logic gates interconnected by a network of interconnect structures. Reliable operation requires that each of the systems and their component pieces be initiated or powered up in a known state. Once a known state has been achieved, the computer system inputs and outputs may then be manipulated and monitored to transition the computer system from one known state to a next known state in a predictable manner. The predictability of the computer system is an essential feature of any state machine and is obviously a cornerstone of reliable operation.
Predictability can be lost, however, if the computer system is operated before achieving a known or initial state. Computer systems may be inadvertently and undesirably forced into an unknown or indeterminate state by a variety of mechanisms. One not too uncommon scenario that can wreak havoc with the various modules of a computer system occurs when the power supply voltage is briefly or temporarily disrupted. Line disturbances and other relatively brief interruptions in the steady supply of power to a computer system can force the system into an unknown state by initiating a power up sequence before the system has achieved a suitable power off condition. Due to the sometimes large system or bulk capacitance associated with computer systems, energy or charge stored in the system may require a relatively long time to fully achieve a zero power or "off" state. If a comparatively brief line disturbance occurs, it is entirely possible that a power up sequence is initiated before the computer has reached a steady state condition. Under these circumstances, the computer system may be in any of an essentially infinite number of states when the power up sequence is initiated. The state of the computer system that results when a power up sequence is initiated under such circumstances can be unpredictable. This unpredictability presents a serious reliability concern to users and makers of computer systems, especially in view of the inevitability of power outages and external line disturbances, which are almost universally beyond the control of the computer user and manufacturer alike. Accordingly, it would be highly desirable to provide a practical solution to assist computer systems in achieving a zero power condition whenever a significant line disturbance is encountered.